Mass spectrometry is an analytical methodology used for qualitative and quantitative determination of chemical compounds in a chemical or biological sample. Analytes in a sample are ionized, separated according to their mass by a spectrometer and detected to produce a mass spectrum. The mass spectrum provides information about the masses and in some cases the quantities of the various analytes that make up the sample. In particular embodiments, mass spectrometry can be used to determine the molecular weight or the molecular structure of an analyte in a sample. Because mass spectrometry is fast, specific and sensitive, mass spectrometer devices have been widely used for the rapid identification and characterization of biological analytes.
Mass spectrometers may be configured in many different ways, but are generally distinguishable by the ionization methods employed and the ion separation methods employed. For example, in certain devices parent analyte ions are isolated, the parent ions are fragmented to produce daughter ions and the daughter ions are subjected to mass analysis. The identity and/or structure of the parent analyte ion can be deduced from the masses of the daughter ions. Such devices, generally referred to as tandem mass spectrometers (or MS/MS devices) may be coupled with a chromatography system (e.g., a GC or HPLC system or the like) and a suitable ion source (e.g. an electrospray ion source) to investigate analytes in a liquid sample.
Certain mass spectrometry systems employ a linear ion trap (otherwise known as a “two-dimensional” ion trap) in order to obtain mass information about ions. The most basic linear ion trap contains four conductive rods arranged to form a quadrupole, and a pair of plates that cap the ends of the quadrupole. Ions are trapped within the quadrupole by an RF trapping field produced by the rods and a DC trapping field that is produced by the pair of plates. In this case, the quadrupole is non-segmented in that it contains a total of four rods. Because of the design of non-segmented ion traps, ions present in a non-segmented ion trap may be exposed to significant non-linear fringe fields. Such fringe fields can excite ions and cause their loss from the ion trap.
The efficiency of linear ion traps has been greatly improved by dividing the quadrupole into spatially separate segments, and linearly arranging those segments in tandem to form a segmented ion trap. Each segment of a segmented ion trap contains four rods that may be, but not always, hyperbolic in cross-section in order to match the equipotential contours of the RF field desired within the segment. Segmented ion traps generally contain from three to twelve segments, although segmented ion traps containing more than twelve segments could be employed in many applications. One type of segmented ion trap illustrated in FIG. 1 contains three segments: a front segment 2, a central segment 4 and a back segment 6. The two end segments differ in DC potential from the central section to form a “potential well” in the center section to constrain ions axially. In this example, a slot in one or more of the rods in the central segment allows resonant ions to be ejected radially out of the central segment in response to a particular RF field applied to the central segment. The ejected ions may be detected using a detector that is adjacent to the ion exit of the slot. By varying the magnitude of the RF voltage applied to rods in the central segment, ions can be ejected in order of their m/z and, as such, an ion trap may be used to determine the mass of unknown ions in a sample.
Because ions are trapped within a segmented linear ion trap in a long, narrow, generally cigar-shaped cloud that may span several segments, segmented linear ion traps are exquisitely sensitive to mechanical imperfections. In order to produce a highly sensitive, high-resolution segmented ion trap, it is imperative to manufacture the ion trap so that the segments are precisely aligned and contain rods that are parallel to each other within high tolerances. For this reason, the manufacture of segmented ion traps presents a unique manufacturing problem. This problem is compounded in manufacturing ion traps containing larger number of segments (e.g., 9 to 12 segments) since systematic errors (rather than random errors) will have more of an effect.
Prior art methods for manufacturing segmented linear ion traps generally involve mounting pre-made rods onto a precision-made insulators using precision spacers and screws. However, such methods are generally very expensive to perform, both in terms of parts and labor.
In view of the above, improved methods for manufacturing a segmented linear ion trap are needed. The invention described herein meets this need, and others.